1. Field of the Invention
This invention relates generally to the continuous, liquid phase oxidation of an alkyl aromatic with an oxygen-containing gas at an elevated temperature and pressure and in the presence of an oxidation catalyst, and more particularly concerns a method for effecting this known oxidation process either at a relatively reduced temperature or pressure or at a relatively increased throughput of the alkyl aromatic.
2. Description of the Prior Art
The liquid phase oxidation of an alkyl aromatic is a highly exothermic reaction. Conventional processes for the oxidation of alkyl aromatics in the liquid phase to form carboxylic acids are generally performed in vertically disposed cylindrical reactors with a substantial portion of the heat generated by the exothermic oxidation being removed by directly evaporating a portion of the solvent and alkyl aromatic in the reaction mixture. The remainder of the heat generated results in an increase in the temperature of the reaction mixture. The temperature of the reaction mixture is determined principally by the total amount of heat generated in the oxidation less that amount of heat removed by solvent evaporation and, except for variances resulting from imperfect mixing of the reaction mixture within the reactor, the temperature of the reaction mixture is substantially the same throughout the reactor.
Because of the great commercial importance of the oxidation of alkyl aromatics, it is highly desirable to improve the yield and quality of aromatic carboxylic acids produced thereby. It has been discovered that the use of lower process temperatures in this oxidation process affords selectivity and product quality benefits. Lower process temperatures favor the oxidation reaction over competing reactions which lead to the formation of undesirable products which reduce the yield and purity of the aromatic carboxylic acids produced. An increased yield of the aromatic carboxylic acid product could be effected by an increased throughput of the alkyl aromatic feedstock through the oxidation reactor.
On the one hand, at a given throughput of the alkyl aromatic feedstock, everything else being equal, the process temperature could be lowered by reducing the process pressure. In that case, increased vaporization of the reaction solvent would occur at the reduced reaction pressure, and relatively greater amounts of the given amount of heat generated by the exothermic oxidation could be removed by the increased vaporization. On the other hand, everything else being equal, increased throughputs of the alkyl aromatic feedstock could be employed if the increased heat generated thereby could be dissipated by increased vaporization of the reaction solvent at a reduced reaction pressure.
However, a serious obstacle associated with the operation of the overhead condenser system must be overcome before the liquid phase oxidation of the present invention could be operated at a lower process pressure. In particular, the material--that is, solvent and alkyl aromatic--vaporized as a result of the heat generated in the exothermic reaction and unreacted oxygen and other components of the air fed to the reactor pass upward through the reactor and are withdrawn from the reactor from a point above the top level of the liquid reaction mixture in the reactor and passed upward and out of the reactor to an overhead condenser system where the vaporized solvent and alkyl aromatic are condensed for recycle by gravity to the reactor. The non-condensible gases are vented from the condenser through a vent.
The overhead condenser system can be made up of one or more condensers; and, if the overhead condenser system comprises a plurality of condensers, typically they are operated in series. Conventionally, the condensed solvent and alkyl aromatic are recycled through one or more lines from the condenser system to the reactor at a point high in the reactor. However, in such a system, pressure drops which develop in the line through which the vaporized material is conveyed from the reactor to the overhead condenser system and through the overhead condenser system itself limit the pressure available to overcome any back pressure at the point in the reactor where the condensed material is returned to the reactor. Under conditions where the process pressure is reduced, both the rate of vaporization in the reactor and the volumetric flow rate of vaporized solvent and alkyl aromatic from the reactor to the condenser system are increased, thereby increasing the pressure drop therein and further decreasing the pressure available in the line to overcome any back pressure within the reactor. Ultimately a point is reached where the pressure drop through the condenser system exceeds the elevation head, and gravity flow of the condensed solvent and alkyl aromatic from the overhead condenser system to the reactor is not possible. This limitation has prevented the benefits from operation of the aforesaid liquid phase oxidations at lower temperatures and pressures and at higher throughputs of the alkyl aromatic from being attained.
It is known in the prior art to return the condensed material from the overhead condenser system to a point high in the reactor through a line external to the reactor and then through a line inside the reactor through the hot reaction mixture within the reactor to a point low in the reactor where the condensed solvent and alkyl aromatic is finally discharged to the reactor. However, the use of a line inside the reactor to bring the condensed material from a point high in the reactor to a point low in the reactor suffers from several disadvantages. Such an interior line can lead to dead zones within the reaction mixture inside the reactor where inefficient and incomplete mixing may occur, and can also lead to the precipitation of solids on the outer surface of the line which would further reduce the mixing efficiency. In addition, since the condensed materials in the interior line are warmed by heat transferred from the reaction mixture along the entire length of the interior line, the potential cooling power that the condensed materials possess as they enter the reactor is not fully utilized in the primary reaction zone in the bottom portion of the reactor. Instead their cooling power is dissipated to a considerable extent by the less efficient indirect heat transfer through the interior line.